Supersaturation and particle dynamics in evaporating vertical falling films

Licentiate thesis, 2025

Crystallization fouling in industrial heat exchangers occurs when dissolved salt species in liquids exceed their solubility limits, leading to supersaturation, nucleation, and subsequent crystal deposition on heat-transfer surfaces. The resulting solid fouling layer increases thermal resistance and reduces system efficiency. This study aims to investigate the onset, evolution, and dynamics of supersaturation — the precursor of crystallization fouling — in vertical falling films to elucidate the coupled transport phenomena governed by film hydrodynamics. A two-phase Direct Numerical Simulation (DNS) framework based on the Volume of Fluid (VOF) method was developed to fully resolve hydrodynamics, heat transfer, and sufficiently capture the very fine scales of mass transport. The simulations show that supersaturation originates at the gas–liquid interface due to interfacial evaporation, while advection and diffusion redistribute it within the film through recirculation zones and local turbulence. The supersaturation dynamics exhibit a pronounced dependence on the Reynolds number (representing the wetting rate of the film), highlighting the strong coupling between the studied transport mechanisms. In parallel, as an ongoing work, a Lagrangian particle-tracking (LPT) framework was developed to investigate the transport of nucleated crystal seeds arising from supersaturated regions. The framework incorporates all relevant hydrodynamic forces, including near-wall corrections to drag and lift. Preliminary results indicate that shear-induced lift governs lateral particle migration toward the wall — a key mechanism for studying particle deposition. This coupled approach links supersaturation and particle transport, providing a foundation for predicting and mitigating crystallization fouling in industrial falling films.